Field of the Invention
The invention relates to a method for magnetic resonance imaging, a magnetic resonance apparatus and a data storage medium encoded with programming instructions to implement such a method.
Description of the Prior Art
In the scanner of a magnetic resonance apparatus, also called a magnetic resonance tomography system, the body of an examination person, in particular a patient, to be examined is conventionally exposed with by a basic field magnet to a relatively high basic magnetic field, for example of 1.5 or 3 or 7 tesla. In addition, gradient pulses are produced by the operation of a gradient coil arrangement. Radio-frequency pulses, for example excitation pulses, are then emitted by a radio-frequency antenna arrangement by suitable antenna devices, and this leads to the nuclear spins of specific atoms excited in a resonant manner by these radio-frequency pulses being tilted by a defined flip angle with respect to the magnetic field lines of the basic magnetic field. As the nuclear spins relax, radio-frequency signals, known as magnetic resonance signals, are radiated that are received by suitable radio-frequency antennas and then processed further. Finally, the desired image data can be reconstructed from the raw data acquired in this way.
For a specific scan, a specific magnetic resonance sequence, also called a pulse sequence, is emitted that includes a sequence of radio-frequency pulses, for example excitation pulses and refocusing pulses, and appropriate gradient pulses that are to be emitted in a coordinated manner in various gradient axes in various directions. At a time appropriate therewith, readout windows are set, which specify the periods in which the induced magnetic resonance signals are detected.
The homogeneity of a basic magnetic field in an examination volume is very important in magnetic resonance imaging by such a magnetic resonance apparatus. Even small deviations in the homogeneity can lead to large deviations in the frequency distribution of the nuclear spins, so poor quality magnetic resonance image data are recorded. For example, magnetic resonance recording methods, such as echo planar imaging, place high demands on the homogeneity of the basic magnetic field.
The examination object itself constitutes a source of inhomogeneity. If, for example, a person to be examined is introduced into the magnetic resonance device, the material of the body interferes with the homogeneity of the basic magnetic field.
To counter this problem it is known to use an adjustable shim unit. By operation of this shim unit, a shim process, known as “in vivo shimming”, can be carried out individually for different examination objects. In this shim process, first a local B0 field in an examination region is conventionally scanned, wherein a B0 field map, also called a B0 field map or B0 map, is created. Constant shim currents (DC offset currents), which flow through gradient coils of the magnetic resonance device, are ascertained using the B0 field map. Furthermore, using the B0 field map currents can be calculated for specific shim channels of a higher order, which can compensate the local field distortions. After adjustment of these currents a resonance frequency is usually ascertained in a frequency adjustment for desired spectral components of an examined tissue, in particular of protons bound to water. The DC offset shim currents through the gradient coils and/or the currents for the shim channels of a higher order and/or the resonance frequency can then form a shim parameter set, which is used for acquiring magnetic resonance image data.
It is known that compensation of local field distortions functions all the more completely the smaller a volume is in which the main magnetic field is to be homogenized. It can therefore make sense to ascertain various shim parameter sets for a plurality of sections of the examination region separately and to adjust the shim unit differently using the various shim parameter sets for acquiring magnetic resonance image data from the plurality of sections. Rapid switching-over of the shim currents and resonance frequency during a runtime of the magnetic resonance sequence for acquiring the magnetic resonance image data on a sequence-dependent timescale of a few milliseconds to a few seconds may be necessary. For this reason, this type of shim process is also conventionally called dynamic shimming. Conventional shimming with a single set of parameters for the whole examination region will be called global shimming below.
Dynamic shimming is described, for example, in the document Blamire et al., “Dynamic Shim Updating: A New Approach Towards Optimized Whole Brain Shimming”, 1996, MRM, 36, 159-165 and in the document Morrell et al., “Dynamic shimming for Multi-Slice Magnetic Resonance Imaging”, 1997, MRM, 38, 477-483.
The accuracy with which local inhomogeneities of the basic magnetic field are compensated depends on the number and order of existing and usable shim channels. In modern MR systems, linear shim terms are usually generated, for example in the three directions, by the DC offset currents through the three gradient coils. Due to the linear field characteristic in the gradient directions, the gradient coils filled with the static shim currents can then be called shim channels of the first order. Furthermore, magnetic resonance devices have dedicated shim coils. These dedicated shim coils can then form shim channels of a higher order, for example of the second order. Shim channels of this kind are often constructed such that the compensation fields which they generate can be described by spherical harmonics. Since the order of the corresponding spherical harmonics is limited, however, it is not usually possible to completely correct fields that vary quickly in the vicinity of susceptibility interfaces. These shim channels can therefore generate compensation fields of a higher order. For example, typical shim channels of the second order comprise five shim coils.
The gradient coils and associated gradient amplifiers are typically designed such that the gradient fields can be varied on a timescale of a few microseconds, for example of roughly ten microseconds. This is usually sufficient for dynamic shimming. By contrast, the field of shim channels of a higher order is typically adjusted only after a settle time of the order of a second or a few seconds. This is frequently not enough for dynamic shimming. The cause of this can be due to the design since faster adjustment of these shim channels demands more powerful amplifiers and/or possibly wires having low resistance. This can increase the costs of the magnetic resonance apparatus. The space that is available to the shim coils of the shim channels of a higher order can potentially also be limited, or the installation of appropriately modified shim channels of a higher order can require a new design or a disadvantageous design of other components of the system.
For these reasons often only the shim channels of the first order, namely conventionally the gradient coils, and/or an RF center frequency, are dynamically switched in a dynamic shim process. Therefore typically only the currents in the shim channels of the first order vary during the runtime of the magnetic resonance sequence. In the shim channels of a higher order a current is conventionally adjusted once before the scan. This current should then be kept constant across the scan. The constant current can correspond to a device-specific tune-up value or, as described above, be ascertained in a specific “in vivo” adjustment carried out in advance for the examination object.